RIA was the forerunner of heterogeneous immunoassays. Difficulties associated with the handling and storage of radioactivity, disposal of radioactive waste and the half-life of the radioactive labels have resulted in RIA being replaced largely by non-isotopic EIA methods.

Two main types of RIA have been used for drug testing based on the isotope employed. An isotopic drug label is termed a tracer. Use of tritium (3H)- or 14C-labelled drug allows an identical molecular structure of tracer to the drug being tested, although counting the beta emissions involves an organic-based scintillation fluid and a sophisticated beta scintillation counter. Use of the gamma-emitting 125I as tracer allows faster and more efficient counting and a number of research (Hand et al. 1986) and commercial RIAs were based on this isotope.

The immunoassay principle for RIA is the same as that for EIA described above. Antibody-coated tubes have been used to good effect with iodinated tracers. Simple decanting easily separates the bound and free fractions, and allows quantification of the bound fraction using a gamma counter. Other ways to separate bound and free fractions include the use of a second antibody precipitation step. This requires a centrifuge to form a pellet of bound material that can be counted after the free fraction has been decanted or aspirated. Magnetic beads and other particles coated with a second antibody have also been used to aid this separation of the bound and free fractions. Dextran-coated charcoal has been used to absorb free tracer from solution, and thereby allow the antibody-bound fraction in the supernatant to be counted (Bartlett etal. 1980).

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